The invention relates to a product having a sensor, by means of which a measurement variable which corresponds to a reactance and which is within a measurement range can be supplied, having a matching network and having a surface acoustic wave element, with the sensor being connected via the matching network to a first reflector in the surface acoustic wave element, and the first reflector together with the matching network and the sensor forming a resonator.
The invention also relates to a method for determining a measurement variable, which corresponds to a reactance, within a measurement range by a sensor, which is connected via a matching network to a first reflector in a surface acoustic wave element, and which, together with the first reflector and the matching network, forms a resonator. The method comprises the following steps:
a) production of a surface acoustic wave which propagates on the surface acoustic wave element;
b) production of a first reflected acoustic wave by reflection of the surface acoustic wave on the first reflector;
c) reception of the first reflected surface acoustic wave; and
d) determination of the measurement variable from the first reflected surface acoustic wave.
The invention also relates to a corresponding arrangement.
A product such as this, a method such as this and an arrangement such as this are described in the article xe2x80x9cSAW Delay Lines for Wirelessly Requestable Conventional Sensorsxe2x80x9d by R. Steindl, A. Pohl, L. Reindl and F. Seifert, IEEE Ultrasonics Symposium, Proceedings, pages 351 et seqq, see in particular FIGS. 1, 4 and 5 together with the associated description. Reference is additionally made to the article xe2x80x9cWirelessly Interrogable Sensors for Different Purposes in Industrial Radio Channelsxe2x80x9d by the same four authors who have been named, 1998 IEEE Ultrasonics Symposium, Proceedings, pages 347 et seqq, see in particular the chapter entitled xe2x80x9cRadio Request Methodsxe2x80x9d, page 349 et seq. Finally, reference is made to the article xe2x80x9cFunksensorik und Identifikation mit OFW-Sensorenxe2x80x9d [Radio sensor systems and identification using SAW sensors] by L. Reindl, G. Scholl, T. Ostertag, F. Schmidt and A. Pohl, presented at the ITG/GMA specialist conference on xe2x80x9cSensors and measurement systemsxe2x80x9d between Mar. 9 and 11, 1998 at Bad Nauheim, a written script of which lecture was provided. FIG. 18 of the script together with the associated description is of particular interest.
The above significant technological background also includes the articles xe2x80x9cSurface Acoustic Wave Filters for Digital Radio Relay Systemsxe2x80x9d by G. Riha, H. Stocker and P. Zibis, Telcom Report 10 (1987) Special xe2x80x9cRadio Communicationxe2x80x9d 241 and xe2x80x9cReproducible Fabrication of Surface Acoustic Wave Filtersxe2x80x9d by W. E. Bulst and E. Willibald-Riha, Telcom Report 10 (1987) Special xe2x80x9cRadio Communicationxe2x80x9d 247. The article xe2x80x9cProgrammable Reflectors for SAW-ID-Tagsxe2x80x9d by L. Reindl and W. Ruile, 1983 IEEE Ultrasonics Symposium, Proceedings, pages 125 et seqq is also of importance.
The technology of autonomous sensor modules which can be checked without the use of wires has developed in recent years on the basis of a requirement for monitoring measurement variables such as wear, pressure and temperature in the tires of a passenger or goods vehicle. A completely passive sensor module comprising a surface acoustic wave element, an antenna and a sensor as well as any matching networks that may be required promises particular advantages for this purpose. A sensor module such as this does not require its own power supply, since the measurement variable which is determined by the sensor can be checked at any desired time by means of a high-frequency pulse transmitted to the module. This is explained in detail in the articles mentioned initially. A sensor module such as this can be checked by an evaluation appliance at a distance of several meters using radio frequency signals from an appropriate frequency band (for example the frequency band around 434 MHz). Possible sensors include a temperature sensor and a pressure sensor, and the sensor module is sufficiently small and compact to allow it to be installed in a conventional automobile tire.
As is evident from the three documents cited initially, the amplitude of the signal which is reflected on the reflector (which is connected to the sensor) of the surface acoustic wave element is the variable to be evaluated for the measurement, and thus governs the achievable measurement resolution. A pressure sensor in particular has a reactance as the measurement variable and can be connected to the reflector via a matching network such that it forms a resonator which allows the amplitude of a surface acoustic wave which is reflected by the reflector to be varied in accordance with the variability of the measurement variable. The evaluation of the amplitude of the reflected surface acoustic wave has the disadvantage that it is necessary to take into account a measurement error which is a function of this amplitude. The smaller the amplitude, the smaller is the separation between the amplitude and the noise, which is always present, and, in a corresponding way, the poorer is the achievable resolution. Since a minimum separation between the signal and the noise (signal to noise ratio) must not be undershot for sensible evaluation, the measurement dynamic range is thus restricted. There is also a restriction with regard to the distance between the evaluation appliance and the sensor module, since the amplitude which can be received by the evaluation appliance falls as the distance increases. In a corresponding way, the present prior art excludes long range measurements and high resolution measurements.
The invention is thus based on the object of specifying a product, a method and an arrangement of the type mentioned initially, each of which avoids the described disadvantages and allows measurement of a measurement variable which corresponds to a reactance, and which measurement is not restricted by the necessity to reach a compromise between the achievable resolution and the achievable range.
In order to achieve this object, a product is specified having a sensor, by means of which a measurement variable which corresponds to a reactance and which is within a measurement range can be supplied, having a matching network and having a surface acoustic wave element, with the sensor being connected via the matching network to a first reflector in the surface acoustic wave element, and with the first reflector together with the matching network and the sensor forming a resonator. For a value of the measurement variable within the measurement range, the resonator has a resonance with respect to a reflection of a surface acoustic wave, which propagates on the surface acoustic wave element, on the first reflector.
In order to achieve this object, a method is specified for determining a measurement variable, which corresponds to a reactance, within a measurement range by a sensor, which is connected via a matching network to a first reflector in a surface acoustic wave element, and which, together with the first reflector and the matching network, forms a resonator, which resonator has, for a value of the measurement variable within the measurement range, a resonance with respect to a reflection of a surface acoustic wave, which propagates on the surface acoustic wave element, on the first reflector, comprising the following steps:
a) production of a surface acoustic wave which propagates on the surface acoustic wave element;
b) production of a first reflected acoustic wave by reflection of the surface acoustic wave on the first reflector;
c) reception of the first reflected surface acoustic wave; and
d) determination of the measurement variable from a phase of the first reflected surface acoustic wave.
In order to achieve this object, an arrangement is specified for determining a measurement variable, which corresponds to a reactance, by a sensor, which is connected via a matching network to a first reflector in a surface acoustic wave element, and which, together with the first reflector and the matching network, forms a resonator, which resonator has, for a value of the measurement variable within the measurement range, a resonance with respect to a reflection of a surface acoustic wave, which propagates on the surface acoustic wave element, on the first reflector, comprising means for:
a) production of a surface acoustic wave which propagates on the surface acoustic wave element;
b) production of a first reflected surface acoustic wave produced by reflection of the surface acoustic wave on the first reflector; and
c) determination of the measurement variable from a phase of the first reflected surface acoustic wave.
According to the invention, the evaluation of an amplitude of a reflected surface acoustic wave is accordingly replaced by an evaluation of a phase of the reflected surface acoustic wave. This requires a specific measure in the sensor module, since the variability of the phase as a function of a reactive measurement variable is at its greatest when the relationship between the amplitude of the reflected surface acoustic wave and the measurement variable is at its lowest. This is the situation when the frequency used for checking is a resonant frequency of the resonator that is formed from the sensor, the matching network and the first reflector. This precludes evaluation of the amplitude of the reflected surface acoustic wave, since the amplitude is no longer uniquely dependent on the measurement variable, at least in a portion of the measurement range. This also means that the dynamic range of the amplitude of the reflected surface acoustic wave is considerably reduced in comparison to the capabilities of the prior art, so that the problem of maintaining a necessary minimum signal to noise ratio is considerably reduced. The dependency of the measurement variable itself on the amplitude that is directly to be measured is also reduced, which means that the measurement error to be considered is considerably less dependent on the measurement variable.
Preferred developments of the invention will now be described; it is self-evident that these are intended for all three embodiments of the invention as described above, namely the product, the method and the arrangement.
One preferred development is for the resonance to be governed by a maximum reflectivity of the first reflector. This maximizes the amplitude of the reflected surface acoustic wave, which considerably improves the achievable measurement resolution.
It is likewise preferable for the resonance to be unique within the measurement range; this also ameliorates any possible low dynamic range of the amplitude of the reflected surface acoustic wave.
The measurement variable is preferably a capacitance, thus corresponding to the choice of a capacitive sensor, in particular of a pressure sensor. In this case, furthermore, the matching network is preferably an inductance connected in series with the sensor.
The surface acoustic wave element is preferably equipped with a second reflector, which is also preferably not switched. The second reflector is used to form a second reflected surface acoustic wave in addition to the first reflected surface acoustic wave, which has been mentioned. This second reflected surface acoustic wave can be used as a reference signal for determining the phase of the first reflected surface acoustic wave. The phase measurement can thus be carried out largely independently of environmental influences.
This is particular important when the sensor is used in a rotating motor vehicle tire. This is because this results in variability of the phase between the rotating sensor and the evaluation appliance which is installed in a fixed position in the motor vehicle, which variability could in some circumstances adversely affect the measurement, but not the measurement using the second reflected surface acoustic wave. It should also be remembered that the measurement can be adversely affected if the temperature of the surface acoustic wave element fluctuates. Any influence such as this can be determined using a second reflector or a number of second reflectors, with the determination of the desired measurement variable being corrected if necessary. A second sensor in the circuit can be provided for an additional measurement for the purpose of comparison, calibration or compensation for an influence which would otherwise be disturbing.
It is particularly preferable for the surface acoustic wave element to have an electroacoustic transducer, to which an antenna is connected. This allows the surface acoustic wave to be produced by sending a pulsed, incoming electromagnetic radio frequency signal to the antenna; the first reflected surface acoustic wave, or any reflected surface acoustic wave, is also transmitted by being converted by the transducer to an appropriate outgoing electromagnetic signal, which is transmitted via the antenna.
In this context, an evaluation appliance which is mechanically separate from the sensor and from the surface acoustic wave element is preferably provided, which has a transceiver for producing the incoming electromagnetic signal to be sent to the antenna and to be converted by the transducer, and for receiving every outgoing electromagnetic signal converted by the transducer, as well as a phase discriminator for determining the measurement variable. This phase discriminator can be configured on the basis of the knowledge of a relevantly experienced person employed for this purpose; in the simplest case, the phase of the received signal can be determined relative to the phase of an oscillator which has produced the electromagnetic signal sent to the sensor. A corresponding phase discriminator can be produced using conventional analog radio frequency technology. Alternatively, the phase discriminator may operate such that it first of all stores a respective outgoing electromagnetic signal both for the first reflected surface acoustic wave and for the second reflected surface acoustic wave, and then compares the two stored signals with one another; this may be done using a signal processor which is based on digital technology and is provided with an appropriate program.